When you create an application code, the apk file contains a .dex file, which contains binary Dalvik bytecode. This is the format that the platform actually understands. However, it’s not easy to read or modify binary code, so there are tools out there to convert to and from a human readable representation. The most common human readable format is known as Smali. This is essentially the same as the dissembler you mentioned.
For example, say you have Java code that does something like
int x = 42
Assuming this is the first variable, then the dex code for the method will most likely contain the hexadecimal sequence
13 00 2A 00
If you run baksmali on it, you’d get a text file containing the line
const/16 v0, 42
Which is obviously a lot more readable then the binary code. But the platform doesn’t know anything about smali, it’s just a tool to make it easier to work with the bytecode.
Dalvik and ART both take .dex files containing dalvik bytecode. It’s completely transparent to the application developer, the only difference is what happens behind the scenes when the application is installed and run.